^'^ ^/ XTbe lUniversit^ ot Cbicaao 



FOUNDED BY JOHN D. ROCKEFELLER 



DIGESTIBILITY OF STARCH OF DIF- 
FERENT SORTS AS AFFECTED 
BY COOKING 



A DISSERTATION 

SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF ARTS AND 

LITERATURE IN CANDIDACY FOR THE DEGREE OF 

DOCTOR OF PHILOSOPHY 



(department of SOCfOLOGV) 



BY 
EDNA D. DAY 



CHICAGO 

1908 



TLbc 'Clniversit^ ot Cbicaoo 

FOUNDED BY JOHN D ROCKEFELLER 



DIGESTIBILITY OF STARCH OF DIF^ 

FERENT SORTS AS AFFECTED 

BY COOKING 



A DISSERTATION 

SUBMITTED TO IHE FACULTY OF THE GRADUATE SCHOOL OF ARTS AND 

LITERATURE IN CANDIDACY FOR THE DEGREE OF 

DOCTOR OF PHILOSOPHY 



(department of sociology) 



BY 

EDNA D. DAY 



CHICAGO 

1908 






^ 



T lie \J .-Ji vef&ity, 
F ? '09 



-!> 



LETTER OF TRANSMITTAL. 



U. S. Department of Agriculture, 

Office of Experiment Stations, 

Washington, D. C, July 15, 1908. 

Sir: I have the honor to transmit herewith and recommend for 
pubhcation as Bulletin 202 of this Office a report of investigations 
on the digestibility of starch of different sorts as affected by cooking, 
conducted by Miss Edna D. Day, at the laboratory of physiological 
botany of the University of Michigan, and later at the University of 
Illinois and at the laboratories of physiological chemistry and plant 
physiology of the University of Chicago. 

The investigations were planned in such a way as to supplement 
the investigations on the changes brought about in food products by 
different cooking processes, which have been carried on as a part of 
the cooperative nutrition investigations of this Office, and during 
the work Miss Day was in consultation with the Director and his 
associates having charge of the details of the nutrition investigations. 

Of special interest and importance from the housekeeper's stand- 
point is Miss Day's conclusion that at least in the case of starch 
itself long continued cooking is not necessary, since it increases 
digestibility only slightly and out of proportion to the time and 
trouble involved, though with cornstarch an improvement m flavor 
was noticed after thirty to forty mmutes' cooking. In solving culi- 
nary ]iroblems the changes brought about in other constituents usu- 
ally associat(Ml with starch in foods must, however, be taken into 
account. 

Acknowdedgment should be made to Prof. F. C. Newcombe, of the 
University of Alichigan; to Dr. Charles Hottes, of the University of 
Illinois, and to Profs. Marion Talbot, A. P. Mathews, and Charles R. 
Barnes, of the University of Chicago, as well as to Dr. H. McGuigan 
and Dr. H. Ilasselbring, of the same institution. 
Respectfully, 

A. C. True, Director. 

Hon. James Wilson, 

Secretary of Agriculture. 

[Bull. 202) 



CONTENTS. 



Page. 

Introduction 7 

Materials, apparatus, and methods used in experimental \v(irk 17 

Experiments on tlie effect of cooking starch 22 

Experiments with raw starch 22 

Digestion experiment No. 1 23 

Digestion expeiiment No. 2 24 

Digestion experiment No. 3 24 

Experiments with starch cooked below the hoiling temperatiu'e 25 

Digestion experiment No. 4 26 

Digestion experiment No. 5 28 

Experiments with starch boiled five minutes 30 

Digestion experiments Nos. 6 and 7 30 

Experiments with starch boiled more than live minutes 30 

Digestion experiment No. 8 31 

Digestion experiment No. 9 31 

Digestion experiment No. 10 '. 31 

Digestion experiment No. 11 32 

Digestion experiment No. 12 32 

Digestion experiment No. 13 32 

Digestion experiment No. 14 33 

Experiments ^\dth starcli cooked in a water bath or double boiler 34 

Digestion experiment No. 15 34 

Experiments on the digestibility of starch by different diastases 34 

Digestion experiment No. IG 34 

Digestion experiment No. 17 35 

Experiments on the digestibility of different preparations of the suiiic 

kind of starch 35 

Digestion experiment No. 18 35 

Digestion experiment No. 19 30 

Digestion experiment No. 20 30 

Additional attempts to identify rose amylose in ])otat() starch 37 

Digestion experiment No. 21 37 

Digestion experiment No. 22 37 

Digestion experiment No. 23 37 

The possibility of reversion in wheat and corn standics 38 

Digestion experiment No. 24 38 

Experiments on the digestibility of tapioca, sago, pearled l)aTley, and rice 

starch 38 

Digestion experiment No. 25 39 

Cornstarch v. wheat starch 39 

Deductions and conclusions 39 

Lliull. 2021 



LLUSTRATIONS. 



Fic. 1. Dia.s^i'am representing Meyer's theory of a four-layered starcli grain . . 

A potato starch grain of hxrge size magnitied 385 times 

Raw starch grains magnified 385 times 

Raw starch grains tliat have been digested 38 hours 

Potato starch magnified 87 times, showing successive stages in paste 

fi )rmation 

(). Starc-h grains fn)ni unboiled pastes 

(6) 

tBull. 202] 



Page. 

9 

10 

22 

23 

25 

2fi 



THE DIGESTIBILITY OF STARCH AS AFFECTED BY 

COOKING. 



INTRODUCTION. 

The digestion of starch has been studied from various standpoints 
and by many investigators, but the subject still offers opportunities 
for investigation. Thus, it has been studied with reference to the 
characteristics of enzym activity; it has been studied as a means to 
knowledge of the composition of the starch molecule, and it has 
been studied from the standpoint of plant and of animal nutrition; 
but a search through the literature of the subject leads to the con- 
clusion that it has been very little studied for the purpose of helping 
solve the housekeeper's problem of how starcln^ foods may best be 
prepared for every day human digestion. 

As starch, next to water, is the substance most abundantly present 
in food, a knowledge of the effect of cooking on its digestibility is 
desirable, both from a practical and a scientific standpoint. The 
following comparative study of the relative digestibility of different 
forms of starch — uncooked, cooked below boiling tcnnperature, and 
boiled for short and long periods of time — is presented as a contri- 
bution to this large subject. 

A brief summary of current theories bearing on the subject may 
fitly precede a report of these investigations, and it is believed such 
a summary will be helpful to the student of starch who has found 
theories varied and conflicting. It will certainly be helpfid in under- 
standing the results of the experiments here recorded and in judging 
of the validity of the conclusions reached. 

The empirical knowledge, gained by generations of experience in 
cooking and eating starchy foods, first deserves attention. Judging 
from popular traditions among housekeepers, such experience teaches 
that (1) raw starch is indigestible or at least relatively so; (2) to be 
made easy of digestion starch should be cooked at boiling tempera- 
ture or higher; (3) after the starch has once been brought to the 
boiling point its cooking may be continued at a somewhat lower 
temperature, as, for instance, in the double boiler; (4) in general, 
the longer starch is cooked the better; (5) in cooking starch lumps 

(7) 
5G551— Bull. 202—08 2 



should be avoided; (6) cornstarch needs longer cooking than does 
wheat starch; (7) of the two starchy foods, rice and potatoes, rice is 
more easily digested than potato, and (S) French cooks believe that 
])otato starch has better culinary properties than cornstarch or 
wheat starch (flour) in thickening sauces and gravies. 

Next to housekeepers, those most interested in this problem have 
been ]>hysicians, and statements from text-books on physiological 
chemistry follow: 

"Tlie individual grain [of starch] is inclosed in a capsule of so- 
called starch cellulose which is insoluble in water but which can be 
made to open by heating in the presence of mucli water. The con- 
tained starch granulose can thus be ()l)tained."" 

Another \\Titer'' states that according to ordinary opinion, starch 
grains consist of two dift'erent substances, starch granulose and 
starch cellulose, of which the fii'st only goes into solution on treat- 
ment with diastatic enzyms. Starch is practically insoluble in cold 
water. Tn warm water tlie grains swell up antl burst, yielding a 
paste. Boiled starch is quickly, and unboiled starch very slowly, 
con\'erted into sugai'. A'arious kinds of unboiled starch are con- 
verted witli (Hllereut degrees of rapidity. 

This theory of the outer, indigestible layer of the starch grain 
gives a reason for cooking starch, but it suggests no value in long 
cooking. It is the old theory of Nageli,<^ publislied in 185S, without 
e^ (Ml the modification inade by Brown and Tleron'^ in 1879. Brown 
and Heron taught that starch cellulose formed the outer la\(>r of the 
starch grain and that, unless it was broken meclianically, as by 
gi'indiug with glass, or dissolved ])y cytase, as in the plant cell, it 
|)revented entirely the digestion of raw starch. However, they 
found that, if the starch was made into a thin paste and treated with 
])lenty of malt extract at a temperature above 45° C, the outer layer 
as well as the inner parts was entirely digested. 

But the ])otanists tell us that the theory of the outer layer of 
starch cellulose has been out of date with them for twelve years, and 
])oint to Arthur Meyer's'' studies of starch grains as authority for 
this statement. As a result of his work he offers a theory of starch 
couiposition which is generally accepted by botanists to-day, which 
in brief is as follows: Starch grains are sphero-crystals, i. e., they are 
luore or less regular spheres composed of a mass of radiating needle- 
shaped crystals, which he calls trichites.^ 

« C. Simon. A Text-Book of Physiological Chemistry. Philadelphia, 1904, p. 73. 
'' 0. llammarsten. Lehrbiicli der Physiologischen Chemie. Wiesbaden, 1907, pp. 
:i2(iand 3-16. 

<' Die Stark ekorner. Zurich. 1858. 
'Mdur. Chem. Soc. [London], 35 (1879), pp. 59(i-654. 
' Uiitersuchungen ul)er die Stiirkekomer. Jena, 1895. 
/1M<1.. i.p. 100-129. 

[IJull. 202] 



Figure 1, which is redrawn from Meyer's report,'* represents dia- 
grammatically his theory of the structure of a starch grain having 
four concentric layers. Figure 2 shows the microscopic appearance 
of a normal potato starch grain showing 20 layers, magnified 385 
times. A comparison of the two will give an idea of the very small 
size of tliese tricliites. 

The layers, he believes, are successively^ deposited one outside of the 
other, beginning with the hilum or point of origin. He suggests that 
the difference in appearance of the different layers is due to a differ- 
ence in the size of the trichites and to a difference in the amount of 
space between them, as shown in the figure. This, he believes, is due 
to alternating dift'erences in growth conditions in the plant cell in 
which the grain was formed, the loose layers being formed by partial 
solution, at the surface, during the night, of the starch deposited 
during the previous day.'' 

The outer layer he regards as no different from the otliers exce])t 
that in some cases it is moi-e dense, due to the fact that the starch is 
gathered after the end of the 
growth i)eriod in the plant, and 
the last growth is slower tlian 
the first.'' 

Some of these t richites, accord- 
ing to his theory, are composed 
of ^r-amylose and some of /^'-ani- 
ylose.'' They are both present in 
ever}' layer tliougli ])erhaps in 
different proportions.'^ In some 
grains part of the trichites are composed of amylodextrin. 

Meyer's /i-amylose corresponds in properties to tliest arch granulosc 
of earlier writers. It stains blue with iodin and is easily digestible. 
It is ins()lul)le in cold water./ In hot water (m° to 80°. C.) ]\Ieyer 
believes that the trichites absorb water and swell to a spherical form, 
and that the ordinary starch paste is an emulsion of these tiny sticky 
droplets.fi' At 138° C. he says that they go into solution in water, 
producing a clear licpiid. 

When a starch paste is exposed to dry air, a skin forms on the sur- 
face. Meyer says that this is due to the sticking together of these 
viscous swollen /i-amylose trichites, and that the}' can scarcely be 
separated again by shaking in hot water.* 

His n-amylose corresponds in a measure to the starch cellulose or 
amylocellulose of others. He says that it differs from ^-amylose in 

"Untersuchungf'ii uber<lieStarkek6rner. ^Ilnd., p. IIG. 

Jena, 1895, p. 107. /Ibid., pp. 14-27. 

ftlbid., i)p. 242-248. olhkl., p. 130, 

clbid., p. 133. /abid., p.13. 
<^Ibid., PI?. 1 and 2. 
[Bull. 202] 




Fig. 1. — Diagram representinf; Meyer's theory of a 
four-layered starch grain. (From his Untersuchun- 
gen iiber die Stiirkekomcr, p. 107.) 



10 



a-- 




lackint;- water of ciystallization. These <^i'-amylose tricliites al)^,i)i'D 
])()iliii^- water very slowly. In water at 138° C. they go into a solu- 
tion in every respect like that of /i-anwlose. With iodin solution the 
solid form stains a faint red color. The solution 
gives the usual blue, 'i'lie solid is digested very 
slowly; its solution very rapidly.'' 

In starch grains of dilVerent kinds A'-amylose 
is ])i'escnt in different amounts. lie reports 0.6 
j)cr cent in potato starch, ().*.) [xu- cent in rice, 
1 j)cr cent in coin, 1.") j)cr cent in wheat, and 
2.5 j)er cent in arrowroot.'' The amount ob- 
j-u.-'.-Apotatostarchgraiu ^.^j^^^^^j j-^.^^j^^ j,^^, ^.^^^^^^ kind of starch -rain he 

of largo sizi', iiuignififd .iv> , . , . . 

tiiiR's: </,iiiiiini or point of fouud (liU'ercd with the method used to isolate 

"'■'^''"- it. He obtained 0.7 to i;> per cent from the 

same sample of ])ot ato starch.' This is one I'cason he gives for beli(>v- 

ing that <^>'-aniylose is (>asily convertible into /j-amylose. and that the 

two ai'c essentially one substance. 

Amylodextrin is a term that has h(MMi used hy dilfei'ent wi'iters to 
designate dilferent substances. Meyer uses it to name the substance 
which is a tirst stage in the digestion of stai'ch in the cold with acid, 
and which stains red with iochn. It is slowly solul)le in cold water 
and readily in warm watei". It is easily changed hy further digestion 
to sugar.'' 

There are some starch gi'ains that slain \vi.\ instead of l)lu(> with 
iodin. From them Meyer claims to have extracted amylodextrin. 
vSometimes he luis seen a red color in starch grains which geneially 
stain blue and he considers 1 he color due to amylodextrin foi'med by 
some digestive pi'ocess.' 

(\)ntrary to the experiences of l>row ii and Heron, Meyei' i-ej)orts 
that unbroken raw starch grains can be (ligesi(>d by ])oth malt extract 
and saliva. This is (juite in accord with his theory of a ])()rous starch 
grain (see fig. 1), any |)art of which can be ))enetrated by enzyms 
without a j)relimin' y breaking of an outer wall. He ex|)lains the 
very slow digestior of raw starch by saying that since the j)()res are of 
less than microsco])ic size, (liffusion inward of enzyms and outward 
of digestion products takes place very slowly. He finds the grains 
that show cracks are much more (piickly digested raw than are solid 
grains. But with either the solid or more or less cracked grain the 
sui'face exposed is very small in comparison with that of an ecjual 
(|uantity of starch substance in the form of a paste.'^ 

According to Meyer, then, starch is very slowly digested in the raw 
form; quickly but not completely if it has been maxh^ into a paste 



" Untersuchungen iitxT die Starkcknrncr. 
Jena, 1895. p]>. t3, 14. 
''I hill., p. •). 
'•11 lid., p. •"). 
[Bull. 202] 



''Ibid.. ])]>. 27 
'II. id., p. S2. 
/Ibid., p. -lA. 



39. 



11 

mth water from 60° to 80° C. (140° to 176° F.), and quickly and 
completely if it has been cooked a long time or heated to lo8° C. 
There is notliing in liis theory to justif\^ the cook's insistence on the 
boiling temperature, nor is there warrant for cooking cornstarch 
longer than wheat starch, since he reports that the latter contains 
more o'-amylose than the cornstarch. Moreover, arrowroot starch, 
wliich physicians often recommend for invalids and young children as 
especially easy of digestion, he finds has more cr-amylose than either 
corn or wheat starch. 

In his book, Meyer reviews and criticises in the light of his own 
extensive observations and experiments the earlier investigations on 
the subject. Since its publication in 1895 several hundred articles 
have been published in scientific journals on the subject of starch, 
but only a few of these need be mentioned here. The theory of the 
spheroKuystalline form of the starch grain seems to be generally 
accepted by those who are familiar with it. 

Kra(Mnor" seems to be the only botanist who has recently insisted 
that the outer layer of lh(> starch grain is dilferent from the 
other layers. He suggests that it may be an anhydrid of starcli 
cellulose. 

Tn 1S97 BiUschli'' described some artificial starch grains pivcipi- 
tated from solutions of starch either by eva])oiiition oi- freezing. 
He said also that the skin that forms on staich ])aste on standing, 
which Meyer considers due to the sticking together of swollen 
«'-amylose tricliites, is of the same nature as tlie ])i-e('ipitat(Ml starch, 
and that they are botii harder of solution and digestion than raw 
starch. 

In 1899 Rodewald and Ivattein' reported the formation of artificial 
starch grains in starch solutions made })v heating starch with iodin 
atl30°*C. 

Recentl}' (1903-1 907) four associated scientists, A. Fernbach, 
E Roux, L. ]\Ia(|uenne, and J. Wollf, have given considerable atten- 
tion to this reversion of starch. They report that a clear ase])tic 
solution of starch |)re|)are(l by heating in an autoclave to I'Mf C. 
passes, on cooling, tlirough a series of retrogressive changes. It first 
becomes Yiko ordinary starch |)aste, in which form it is easily digested. 
From tliis form it passes by degrees into a final form, which they call 
amvlocellulose. Tiie characteristics of this form are tliat it drives no 
color with iodin and is difficult both of solution in boiling water and 
of digestion by diastase. Furtliermore, on treating with j)otassium 
hydroxid or by heating again with water to 180° it dissoh^es, forming 

"B()(. Guz., 34 (1902). pp. 341-354. 

''Verhaudl. Naturhi.st. Mod. Ver. lloidellx-rg, 5 (1897), i)p. 457-472. 
''Sitzuugsber. d. Berl. Acad., 33 and 34 (1899), ]). G2S. 
[Bull. 202] 



12 

a solution that colors blue with iodin but that does not gelatinize on 
coolinu: as does starch paste." 

They also report tliat tlie same reverted starch or amylocellulose is 
formed if starch pastes are liquefied by the first stages of digestion and 
then the digestive process checked/ This suggests the descri])tion 
given by Brown and Heron in 1879 of a preci])itate of starch cellulose 
from a starch paste digesting with malt extract in the cold. Brown 
and Heron reported two forms of starch cellulose, one more difficult 
of solution than the other. '' 

Maquenne and Roux'' conclude fi'om their work tliat amylose exists 
in a series of convertible foruis that may be regarded as different con- 
densation forms of the same fundamental nucleus. Amylocellulose 
stands at one end of the series and granulose at the other. Cold and 
concentration of solution tends to change tlie starch to the amylo- 
cellulose forui, and heat and dilution to the granular form, but the 
change to the amylocellulose form takes place oidy when tlie granu- 
lose has first been dissolveth In addition to tlie different forms of 
amylose tliere is, they believe, another substance ])resent in natural 
starch grains which they call auiylo|)ectin. It is the colloidal sub- 
stance which causes starch pastes to gelatinize. It does not stain 
with iodin and it is closely united with the amylose, and while in this 
combination prevents reversion. The two are separated by solution, 
whereupon the amylose reverts, and the reverted form containing no 
ainylopeclin does not gelatinize aft(>r nOieating with water. The 
isolation of amylop(M'tin has not yet heen reporteiL (See p. Ki.) 

This work on reverted starch suggests some new problems in starch 
cooking. It would seem that the more thoroughly starch is cooked 
the more (hmger there is of its reverting to its most indigestible, i. e., 
insoluble form, if it is not consumed at onc(\ 

It occurred to Roux '' that j)erhaps reversion took ])lace as bread 
))ecanie stale, l)ut lie was unable to find any evidence of the fact, 
altiiougii two years before Lindet,' working apparently without this 
theory, had reported iinding the starch in stale bread harder to dis- 
solve than that in fresh bread. 

These four scientists seem to have overlooked an article by Syn- 
iewskif on the constitution of starch, which ap))eared in 1.S99, in 
which the author con(dudes that the amylocellulose of Brown and 
Heron (<^i'-amylose of Me^^er) does not exist in the natural starch grain, 

" ('(.inpt. Kend. Acad. Sci. [Paris], 137 (1903), pp. 12()6-12G8. 

'M'ompt. Rend. Acad. Sci. [Pari.'^], 138 (1904), pp. 819-821 : 139 ( 1904 i, ])p. 1217-1219. 

<• Jour. Cheni. Soc. [London], 35 (1879), pp. 596-654. 

<« Compt. Rend. Acad. Sci. [ParL^], 140 (1905), p]). 1303-1308. 

f (\nn\A. Rend. Acad. Sci. [Paris], 138 (1904), pp. 1356-13.58. 

/ Rul. Soc. Chim. Paris, 3. scr., 27 (1902), pp. 634-6.39. 

9 Liebig's Ann. Chem., 309 (1899), pp. 282-315. 

[Bull. 202] 



13 

but is formed as a reversion product when for any cause starch goes 
into sohition and is not kept at a high temperature or quickly digestecL 
The reasons he gives for these conchisions are (1) that the methods 
they employed to isolate this substance were such as would allow of 
reversion; (2) they obtained varying quantities of it b}^ using different 
methods, more or less favorable to reversion; (3) they give no satis- 
factory explanation of the fact that under favorable conditions starch 
paste is as completely digested as it would be if it contained no 
indigesti})le material, and (4) he analyzed the substance prepared as 
directed for «'-amylose and found that it is a hydrated form of starch, 
instead of starch anhydrid or a condensation product of the same, 
as suggested by others. Its formula he gives as (C6lIio05),jn+nII.X). 

Since Syniewski wTote before the theory of amylopectin he natu- 
rally said nothing about it, but his theory easily explains without its 
help the fact that the reverted product does not gelatinize after 
reheating with water. The starch molecule has decomposed beyond 
the gelatin-forming stage before it reverts. Syniewski gives in this 
article and in one « that followed it a very interesting theory of the 
structural formula of tlie starch molecule and of the various changes 
that come to it on boiling with water and digestion with enzyms. 

Tiong boiling with water, he says, results in a series of hydration 
compounds unlike those given by digestion. Because of the place in 
the molecule where the changes take place, tliese compounds give the 
characteristic blue color with iodin, and do not reduce Fehling's 
solution, as do the prochicts of digestion. These compounds he calls, 
because of their blue color with iodin, amylodextrins (starch-like 
(h^xtrins). ITe has analyzed two of these. The hrst is the compound 
formed wh(>n l)y long boiling or by heating in an autoclave the starcli 
first forms a clear solution, lie gives to it tlie formula (CglliijOj)^,, + 
ouHoO. It is this compound, lie says, which easily reverts. There- 
fore he calls the product ''reverted amylodextrin " instead of re- 
verted starch. Its fornuda is (C,JIi„0-),jn-|-nir,0. The second of 
these auiylodextrin C()mi)ounds which he has analyzed he says is the 
final compound formed by very long boiling with water. He calls it 
amylogen, and gives its fornuda as (Collio0,r;)9 + .311,0 or CjJI,,/)^^- Ih' 
Ixdieves that the starch molecule is an anhydrid condensation product 
of four amylogen grou|)s, which the long ])oiling sei);irates into 
amylogen molecules. 

Syniewski agrees with other writers that by digestion with either 
saliva or malt extract the hrst stage is a change to a clear solu- 
tion, i. e., the ''soluble starch" of many writers and called by 
Syniewski "first amylodextrin." After this blue-staining com- 
pound, all agree that by continued digestion a mixture results that 
contains some maltose and a compound staining red with iodin. 

a Liebig's Ann. Chem., 324 (1902), pp. 212-268. 

(Bull. 202) 



14 

Syniewski calls this maltodextrin I. The hydration, he says, results 
in breaking one molecule of maltose from each amylogen group in the 
starch molecule. The next step results in the separation of more 
maltose, another molecule from each amylogen group, and a com- 
pound staining brown with iodin, his maltodextrin TI. The term 
erythrodextrin is often applied to both these red and brown dex- 
trins. The separation of a third molecule of maltose from each 
amylogen group, he says, results in a compound giving no color with 
iodin, formed of the resi(hie of these four groups. It is very slow of 
further digestion, l^ro vti and Heron called it ''stable dextrin." 
i\h)st writers now agree that it is fmally converted into maltose and 
glucose. 

If Syniewski is right in believing that there is no substance present 
in raw starch that is not changed to an easily digestible form by 
ordinary paste formation, there would seem to be no value in boiling 
starch a so-called "long time," for the more nearly the paste becomes 
a solution the more danger is there of reversion, if digestion can not 
follow al once. Moreover, this change to a .soluble form, which it 
takes hours of boiling to i)roduce, can be accompli.shed by the first 
minute of normal digestion which is followed at once by further 
changes tlnit carry the j)r()cess beycmd danger of reversion. 

Being a chemist :ind not a inicroscopist , Syniew.ski was apparently 
not interested in the j)hy>ical changes that occur in paste forma- 
tion — that is, whetiier outer layers burst or trichites swell. He 
worked with potato starch and has notiiing to say on the relative 
digestibility of dili'erent kinds of starch. 

As far back as 1S74 1). Levberg" reporte 1 that diU'erent ((uantities 
of saliva were necessary to i)ro luce the maxinnim (piantity of glucose 
in the same time of igestion. He oi)laine 1 from potato starch, with 
!) cubic centimeters of saliva, 00.8 per cent glucose: from arrowroot 
starch, with •> cubic centimeters of saliva, .')'.).(')_' p(>r cent glucose; 
from rice starch, with 10 cubic centimeters of saliva, 50.70 per cent 
glucose: and from wheat starch, with IC) cubic centimeters of saliva, 
()"_'. S7 per cent glucose. By varying the experiment he found that it 
took S hours to produce the maxinuun result from arrowroot starch, 
hours from potato starch, 12 hours from wheat starch, and 14 hours 
from rice starch. In both cases it will be notice. 1 that the potato 
and arrowroot starches showed greater ease of digestion than the rice 
and wheat starches. 

In 1896 vStone'' com})ared the digestibility of diiferent kinds of 
starch paste when acted upon by various forms of diastase as shown 
l)y the time it took for the digestive mixtures to reach a stage where 
they could no longer color iotlin solution. He found that when 
treated with malt extract, potato starch required 12 to 15 minutes, 

«Inaug. Diss., St. Tetersburg, 1874. ^Expt. Sta. Record, vol. 8, p. 662. 

IBuU. 202] 



15 

wheat starch 60 to 90, and maize (corn) starch 90 to 120 mmiites. 
When sahva was used potato starch requu"ed 3 mmutes, maize starch 
90 minutes, rice starch 165 minutes, and wheat starch 170 minutes. 
With commercial pancreatin, potato starch required 58 minutes, 
maize starch 337 minutes, while with both rice and wheat starch the 
digestion was not complete in 10 hours. As an explanation of the 
differences wdiich were observed in digestibility Stone suggests that 
starches, like sugar, possibly exist in different isomeric forms. 

In 1904 O'Sullivan" reported that "under similar conditions of 
hydrolysis with diastase * * * tl\e results obtained from potato 
starch show no (juantitative i-elationship with those obtained from 
the other starches." lie did not say what these other starches were, 
but he probably did not use arrowroot starch. 

U. Butyazin,'' in 1S87, reported that of rice, ])eas, millet, and 
buckwheat imder the same conditions of cooking and digestion, millet 
was the most easily digested, then buckwheat, rice, and peas. This 
author also concluded from his investigations that cooked starch 
was more digestible than raw starch, and that the digc^stihility 
increased with iho length of time of cooking. 

In this connection some Japanese work reported by S. Kano and 
S. lisliiina'^ is of interest. These investigators were themselves the 
subjects of tests undertaken to study the digestibility of katakuri 
starch made from KviitJironium clens-canis and kazu starch made 
from Pueraria thunhergldna. Aj)})arently tlie starch was cooked and 
was the sole article of (Het. The coellicient of (hgestibility of the 
katakuri starch was 95.7 per cent and of the kazu starch 96. G j^er 
cent, vahies which agree very closely. 

The digestii)ility of konnyaku, a f(-od product ])repared from the 
tuber of lli/drosme I'ivierl (ConophaUus konjak), wliicli contains man- 
nan as its princi{)al carbohydrate instead of starch, was also studied, 
the coellicient of digestibility of the carbohydrates being 82 per cent, 
or considerably lower than the starches. 

Juilging from tlie work of O'Sullivan and others there is a difference 
in tlie digest il)ility of (Ulfeivnt staiches, but there would seem to be 
more (HfTerence betwecMi ])otato and ''Other starches" than between 
coin and wheat starches. 

From the confusion and ct)ntradictions a few points stand out. 
Thus, all authorities agree that starch paste is very much more easily 
digested ihau raw starch. 

Investigators give no explanation of the cook's insistence on the 
boiling temperature in cooking starch. In fact, in j)rcparing their 
pastes foi- experiments they put no special emphasis on reaching the 

ffjour. Cheni. Soc. [London], 85 (1904), p. GIG. 
'^ Inaut!;. Diss., St. Potersburo;, 1887. 

<- Bui. Col. .Vgr., Tokyo Imp. l^niv., 2 (1891), No. 2— U. S. Dept. A.sjr., Oflico of 
Experiment Stations Bui. 159. 

56551--Bull. 202—08 3 



16 

boiling point or on the length of time that the pastes are cooked. 
The length of time that a paste stands after it is made, or the tem- 
perature at which it stands, ov the presence of traces of impurities 
in the starch are all emphasized as affecting the digestibility, but 
no one seems to ha^'e comj)ared the digestibility of starch cooked 
below boiling temperature with that actually boiled. Some inyesti- 
gators ])ring their starch to a boil, others cook a few moments on the 
water bath after boiling, and others cook on the water bath appar- 
ently without any boiling. Their only effort in this direction seems 
to he to secure a homogeneous paste. 

Inyestigators as well as cooks use care to ayoid lumps in cooking 
starch. 

Many inyestigators use the autoclaye with a temperature of 130° 
to 140^ C. in pre])aring material for e.\periments; for since the pub- 
lication of Meyer's i-esults all agree that the entire starch grain goes 
into solution at that temperature. 

The exj)ression "long boiling," as used l>y inyestigators, is less 
dt'linite than the cook's "pinch of salt,'' but they all agree that it 
has the same effect as heating to 130° to 140° C. in changing the opal- 
escent ])aste to a clear solution. 

But since those who hn\o expcriniciUed with reyerted starch agree 
that starch solutions tend to reyert to forms nu)st didicult of diges- 
tion, long cooking would seem to be of (|uestionable yalue. 

There is a general agreement among investigators as to the pi'oj)- 
erties of the substance that foinis at least the chief mass o{ starch 
grains, but there is nuich disagrcMunent as to the presence in the 
stai'ch grain of one or moi'c other substances (such as rainiose, starch 
cellulose, ^»'-amylose, amylodextrin. and auiylo|)ectin^. and of the 
respect ivt> pi'o{)eities of each. 

Theie is an eyident (liscre})ancv between the beliefs of practical 
cooks and the yarious theories of different scientific investigators. 
In order to find, if possible, an explanation for this fact and to secure 
nioic definite knowledge as to the effects of cooking on the digesti- 
bility of starch, the experiments repoi'ted in tlie following pages were 
un(l(M-taken. 

Since tlu> investigations repotted in thi> hulletin were coin])leted 
and the report was prepari^l for })ul)lication seyeral articles by 
othei' investigators haye a|)i)e;i red whicli hav(> a bearing on the 
conchisions di-nwn. The following ])apers seem especially interesting 
in this connection. 

Mnie. Gratin-Gruzewska" claims to haye isolated the amylopectin 
of Miuiuenne and Roux (see [). 12) from })ot:ito starch l)y the use of 
alkali. She finds by the niicroscoi)e that the substance is composed 
of the outer layers of the starch grains. It stains red with iodin. 

"Conipt. Rend. Acad. Sci. [Paris]. 146 (1908), pp. 540-542. 

[Hull. -'112] 



17 

In wami water it forms a gelatinous paste wliile the solution from 
which it is separated does not gelatinize. Tliis so-called amylopectin 
is the same as the red amylose of this paper. That the blue amylose 
from which it is separated does not gelatinize is, perhaps, due to the 
fact that the alkali has hydrolyzed it beyond the gelatinizing stage. 

England" separates thdn, well-boiled pastes of corn and Inirky 
starches into three layers by using a centrifugal machine and then 
allowing them to stand for twelve hours. The first or upper layer, 
he finds, consists of a clear solution that gives the characteristic blue 
color with iodin and does not gelatinize on standing. He calls it 
soluble starch. The second layer consists of a white precipitate 
which stains a reddish color with a little iodin, a purple with more, 
and H deep blue if still more iodin is added. But if this precipitate 
is thoroughly washed with water to free it of soluble starch it gives 
the red(hsh color alone. On boiling with water it forms a gelatinous 
paste. lie calls it insoluble starch. The lowest layer, when well 
washed, is pearly white in ai)pearance and gives no color with iodin. 
He consiilers that it consists of the broken envelopes of starch grains 
and calls it cellulose. His insoluble starch probably consists of both 
rose and red amylose and his cellulose of reverted amylodextrin. 

Of more i)ractical interest is the work of Harcourt,'' who has 
recently tried some natural digestion experiments on the effect of 
cooking rolled oats and wheat farina. He reports that 98 per cent 
of the'^carbohydrate of the rolled oats was digested if cookc^l 20 
minutes, while only 0.3 per cent more (98.3 per cent) was digested 
if the oats were cooked 8 hours. With the wheat farina 98.3 per 
cent of the carbohydrate was digested if the wheat was cooked 20 
minutes, only 0.2 per cent more (98.2 per cent) if first cooked 8 
hours. Thus, with natural as well as artificial digestion and with 
starch combined with the other constituents of the cereal as well as 
with isolated c(>real starch, there is found to be a slight, but only a 
slight, increase in digestibility secured by cooking for a long time. 

MATERIALS, APPARATUS, AND METHODS USED IN EXPERI- 
MENTAL WORK. 

The investigations here reported were carried on at the University 
of :Michigan and lat(>r at the University of Illinois and at the Univer- 
sity of Chicago 1903 to 1906. 

The materials used in these experiments were potato, wheat, corn, 
arrowroot, and several other forms of starch, sahva, malt diastase, 
and taka diastase. 

f The Cereal Modification of Cow's IMilk. [Philadelphia]. ( 1907). 
f* Ann. Ri)l. Ontario Atrr. Col. and Expt. Farm, 32 (1906), p. (Ki. 
[Bull. 202] 



18 

In the case of potato starch a commercial sample and several 
samples prepared in the laboratory were used. The commercial 
prejinration was neutral in its reaction and quite free from solid 
imj)urities, and therefore no attempt was made to purify it. For 
the laboratory preparations new and old potatoes were bought the 
lirst of May. The old ones were in very ^ood condition for the time 
of the year. The new ones were of <i'ood size. Both kinds were 
washed. ])ared. o;rated under water, and the o;rated material filtered 
throuu'h two tliickness(>s of cheese cloth. The iilterinu' removetl tlie 
cellulose. To rid the starch of the soluble im])iirities, it was washed 
many times 1)V decantatiou. first in tap water and then in distilled 
water. It was di'icd on un<2;la/.ed porcelain ))lates below ()()° C. 
Some of the old jiotatoes were saved and kept in the laboratory 
locker until the hrst week in July. wluMi they were found to be nuich 
sprouted and shriveled. Aftej' renioNiuL:; the s|irouts, starch was 
pr(»])are(l from these potatoes hy the same method. 

In the case of the cornstarch, and wheat starch also, connnercial 
sainpl(>s and laboratoiy |)reparations were used. It was found that 
there were dilVerences in the digestibility of the commercial pi'cpara- 
tions of cornstarch. Of i\w three sani])les tested and desionated 
"A," *'B." and "(\" respectively, "A" and "B" W(>re j)ut up for 
culinary use by dilferent firms and both contained solid inij)urities, 
lafii'ely cellular material. '"A" was so alkaline that it nnpiii-ed ().S6 

cubic centimeters IK'l solution to neutralize 1 ijrani of it, using 

phenolplithalein as an indicator. "B" was wry sliuhtly acid in its 
i'(>a('tion. I)otli were washed with tap watei' until neutral, and then 
with distilled water a nund)er of times and dried on unglazed ])orce- 
lain |)lates. ■"C" was bought from a chemical supply house (under 
the name of wlu'at starch, though it was really coi-nstai-ch) and 
seemed pure enough to use as it was. 

Cornstarch was ])repai'ed in the laboratory by washing it from corn 
meal by the method used with the grated potatoes. The product 
was not pui'e. But it seemed much inoi'c important to have the 
starch as nearly as possible in its natural condition, although mixed 
with im|)urities. than to use any methods to get rid of these that 
might change the comj^osition (»f the stai'ch. 

Some diiliculty was experienced in obtaining a connnercial pre])ara- 
tion of wheat starch. Twice coi-nstarch was received from dealers 
in response to orders for wheat starch. The sample which was 
iinally obtained was used in the condition in which it was received. 
An impure starch was prepared in the laboratory from wheat Hour 
by the method used for cornstarch. In this case, too, the desire to 
kee|) the starch as nearly as ])ossible in its natural condition ])revented 
further ('(fort at ])urific-ation. 

[J5ull. ■J()2] 



19 

A commercial preparation of arrowroot starch was bought at a 
drug store in a tin-foil packag<\ It seemed to be very pure and was 
used as purchased. ^ 

The other forms of starch used were pearled l)arley, ordinary rice 
grains, tapioca in tlu-ee forms, and sago of two sorts. These were 
bought at a grocery store and used in c[ualitative experiments \\'ith- 
out any attempt at purification. 

As previously stated, the ferments used in the experimental work 
were malt diastase, taka diastase, and ptyalin of saliva. In the case 
of malt diastase, a sufficient quantity of a standard preparation from 
barley malt was purchased at the beginning for use in all the experi- 
ments here reported. A homogeneous mixture was made, wliich was 
kept closely corked in a dark-colored bottle. The taka diastase used 
was also the usual commercial preparation. The saliva was collected 
fresh for each set of experiments in which it was used. 

The apparatus used included a microscope and l)urettes, pipettes, 
flasks, etc., such as are needed for ordinary volumetric work. 

The hot-water stage for the microscope, mentioned on page 26, 
was made from a flat bottle of clear glass, which was fitted with a 
cork with two holes. Tlu'ough one passed a l)ent glass tube, the 
other end of wliich dipped in a hot-water bath, and througli tliis tube 
hot water was siphoned into the bottle. A glass tube through the 
other hole in the cork allowed for the exit of water. 

The water l)ath used for keeping the digestion mixtures at a con- 
stant temperature was made double. The outside was a largo 
galvanized-iron can, such as is used for garbage, holding 55 liters of 
water. Inside of this was a large deep can 10 inches less in diametci 
liolding about 13 liters. Tliis was supported by means of ten wires 
])assing from the rim of the iimer can to that of the outer, as the 
spokes of a wheel from a hollow hub to the rim. Thermometers were 
kept in both the inner and outer parts. A small gas flame below 
supplied the heat. An attemjit was made at first to regulate the 
temperature by means of a mercury thermostat, but this ])roved 
unsatisfactory. It was found simpler and better to have the burner 
connected directly with the gas fLxture, with a long slcuider rod 
attached to the stopcock. In turning the gas up or down the end 
of this rod described an arc of a large circle. The point on tlus arc 
at which the rod rested, when under ordinary contlitions the temper- 
ature of the inside of the bath was 55° C, was marked on the table. 
With this normal position determined the gas coukl safely be turned 
up or down as needed, and the right temperature being obtained it 
was found easily possible to keep it constant within one-quarter of a 
degree for the important first half hour of digestion. In long experi- 
ments the t(>mperature often varied two or three degrees chu'ing the 
time when the changes could be noted. 

[Bull. 2021 



20 

Goiierallv speakinjr, the difrerent sorts of starcli tested were made 
into a j)aste witli distilled water before treatment with diastase. Tn 
niakinu' the paste the starch was dried overnight on wei*j;lied wateh 
crystals in a calcium chlorid desiccator, with approximately 1 ijram 
of starch on each watch crystal. This (juantity was then cpiickly 
adjusted to the exact weijj;ht of 1 o;ram, on a delicate balance, and 
then carefully transferred to an P^hrlenmeyer flask of 170 cid^ic 
centimeters capacity and shaken with ."> cubic centimeters of cold 
distilled water. To this was quickly added in three parts 45 cubic 
centimeters of boilinix distilled water, the mixture beins; shaken after 
each additic^n. The pre|)aration of all j^astes for quantitative work 
was hejxun in this way, since it involves no risk of losing any of the 
starch, as does the method ordinarily described of j^ourino; the mix- 
ture of starch and cold water into the boiling water. The further 
])rej)aration of the pastes used in the work reported on pages 25-30 
consisted in cooling them to 55° C, the temj)erature at which the}" were 
digested. Other ])astes were either boiled or cooked in the boiling 
water bath the indicated length of time, a return condenser being 
used in all cases to keep the concentration constant and to prevent 
skin formation. To prevent lumping, the pastes to be boiled were 
l)r()ught to the boiling temperature with constant shakings before 
attachment to the condensei-. After cooking the given length of 
time, the pastes were corked and cooled to 55° C, and by means of 
wires around the necks of the flasks suj)ported inside of the inner 
part of the constant temj^erature water bath. 

For digesting the starch, nnilt extracts were madi^ fresh evervdav. 
Three and a half grams of the commercial malt ])owder was mixed 
with 175 cid)ic centimeters of distilled water at n.'}° C. and kept at 
that temperature for an hour, being shaken every 15 minutes. The 
solution was then filtered with the aid of an as|)irator and heated 
again to 55° C. Twenty-live cubic centimeters of this extract meas- 
ured with a |)ipette was added to 50 cubic centimeters of {\w starch 
jiaste in each llask. 

Taka diastase extract, which was used for |)ur|)oses of comj)arison, 
was prepare<l in the same way as the malt extract, excejjt that the 
])owder was used in the proj)orti(^n of 4, instead of 5, grams to 100 
cu])ic centimeters of water. It is well known that many sj)ecies of 
fmigi produce enzyms capable of inverting sucrose and in some cases 
of converting starch or cellulose into sugar. Taka diastase is a dias- 
tase of this sort developed by the fungus Eurothim oryzx."^^ In a few 
of the experiments saliva was used. 

Two methods were used to determine* the <\\t(Mit ^4 the action of 
diastase, namely, a colorinietric method in which iodin was used as 

"Ztschr. S])intusiiulus.. 18(1895). p. 223. 

[Hull. 2(12] 



21 

a reafrent and a direct determination of the su<:;ar formed ^vith 
Folding's scjliition as a reagent. 

The iodin test indicated not only the rai)idity of the changes taking 
})lace, but something of the products present in the digestion mix- 
ture at any given time. For this test ghiss tubes were kept in the 
flasks during digestion for the removal of samples. At stated uiter- 
vals a small (pumtity was transferred to test tubes and dilute iodin 
solution added. This stopped digestion, and the tubes bemg set 
aside in order, were later carefully compared and notes taken, more 
iodin being added as needed for color determination. It was found 
to be unimi)ortant whether or not a defuiite amount of the digestion 
mixture and t\w iodin were combined, for varying amounts of reduc- 
ing by-j)roducts decolorized the soluticms, and the tubes standing dif- 
ferent lengths of time showed different colors of varying intensity. 
Moreover, it was found that if at the conclusion of the experiment 
weak iodin solution was added slowly in excess all color changes 
could be easily recognized, and the i)resence of an}' b^o^\^lish hue 
due to dextrm could be distinguished from the yellow of the iodin 
solution by fmally comparing the color in the tube with that of the 
same sized tube containing an ecpial amount of the iodhi solution 
with water instead of digestion mixture. This method was found 
very satisfactory. It could not. however, tell all the story, and 
needed to be sui)})leniente(l by the sugar test. 

The sugar test is not of so much A'alue in recognizing the delayed 
digestion of small i)arts of the starch as is the iodin test, but it sup- 
plements it by giving more defmitc means of comparing the digesti- 
bility of the mass of the starch. 

With this method the flasks containing the starch and diastase 
were kept stoppered and shaken every 10 minutes. Digestion was 
allowed to contini^e 30 minutes. Then the flasks were removed 
from the water bath, heated until the contents boiled to stop enzyni 
action, filtered, the wash waters being added to the solution, and 
the whole made up to the mark in a 250 cubic centimeter flask. 
The solution was then tested for sugar by the following modification 
of a method of Lintner's: 

Into test tubes of 40 cubic centimeters caj)acity was run from 
burettes 1 cubic centimeter of each of the two parts of Fehling's solu- 
tion, varying amounts of the digestion mixture, and enough distilled 
water to make the whole up to 15 cubic centimeters. The tubes 
were then j)ut in order into a wire holder and together plunged mto 
a boiling water bath, and again removed together at the end of 
twenty minutes' boiling. The amount of the given sugar solution 
needed to reduce the 2 cubic centimeters of Fehling's solution was 
then ascertained l)y comparison of the tubes. This stage could be 
determined only ap[)roxinuitely at first. After thiee or four hours' 

[Bull. 202] 



09 



standing, the red preci])itate settled and the hist of the bhie color of 
the copper sulphate could be recognized. The end point under 
these conditions corresponded to 0.01 gram (^f maltose for every 2 
cid>ic centimeters of Fehling's solution. From the calculated result 
was sul)tracted the 0.287 gram of sugar found in 25 cubic centimeters 
of the rualt extract used. This method was found to be much the 
most satisfactory of the methods tested involving the use of Fehling's 
solution. 

EXPERIMENTS ON THE EFFECT OF COOKING STARCH. 
EXPERIMENTS WITH RAW STARCH. 

vSonie preliminary experiments were made \\ illi law starch in whicli 
the microscopic apj)earance and its reaction \\itli ioihn wcr(> stu(nc(l 
and (hgestion experiments were ma(k\ 

As is well known, raw starch grains from (hll'creiU kinds of phints 
vary in tlicir microscopic appearance*. 'I'hc potato and arrowroot 






h 



Y\i:,. :i.— Uuw stan^li Kniins maguified ;iSo times: a, from potatu: b. from arrowroot; c an<l (/. from wheat— 
c side view, d flat view; < and/, from liarley: </. from corn (maize); /i, from rice. 

starch grains arc usually irrcgulai'ly ovoid solid grains, showing no 
cracks (see fig. o). The wh(>at and l)arlev starch grains are lens 
shajx'd, generally showing a hollo\V(>d space in the center also lens 
sha|)(Ml. Jlice and corn starch grains are generally i)olyhedral in 
sha[)e, with a more or less star-shaped crack in the center of each. 
As preliminary to the exj)erimental work the diflVrent starches were 
examiiKMl with a microscope and compared with standard samples 
as a means of establishing their identity. 

Ivaw starches of different origin show dillerences when treated 
with a .solution of iodin. When iodin solution is added to raw- 
starch grains a dark pur{)Iish blue results. The })otato and arrow- 
root grains showed but little of the ))m'ple, while the other starches 
which were used showetl on standing a decidedly red-jjurple color. 

[15iill.L'(l2J 



9^ 








DIGESTION EXPERIMENT NO. 1 . 

In this experiment potato, arrowroot, corn, wheat, barley, and rice 
starches were used. One-half gram of each kind of starch was })laced 
in a flask and to each was added 25 cubic centimeters of cold distilled 
water, 3 cubic centimeters of fresh untiltered saliva, and a few drops 
of thymol solution. The flasks were stoppered and put in the constant 
temperature water bath at 40° C. The samples were tested \\'ith the 
naked eye and with the microscope at the end of 14 and 38 hour 
periods. 

With the naked eye no appreciable diminution in the quantity of 
solid starch in the bottom of the flask was noticeable at either time. 

At the end of 14 hours neither the potato nor arrowroot grains 
showed any change when examined with a microscope except a slight 
swelling. In most of the corn and rice starch grains the center was 
apparently all dis- 
solved, leaving be- 
hind a thick shell 
which showed long 
deep clefts along 
what were the 
edges of the solid 
polyhedral g r a i n 
(see fig. 4, a). In 
the case of wheat 
and l)arley starches 
many of the grains 
showed no change 
except corrosion at 
the surface, but 
many others were 
notliing l)iit hollow 
shells, tliinner than in the corn, with pitted surfaces and cracks 
around the edge nuich like the openings of oyster shells (see fig. 4, h). 
At the end of 3S hours many arrowroot grains showed surface cor- 
rosions (see fig. 4, c). Some potato starch grains showed the same 
kind of corrosions. Others showed a change at the small end (see 
fig. 4,(/). While the rest of the starch grain stained a dee}) l)hie with 
iodin, the small end did not stain at all, but api)eared to be of a 
colorless glistening substance less solid than the rest of the grain and 
showed the same laminated appearance. The corn, ric(\ wheat, and 
barley starch grains appeared j)ractically th(> same as the day before. 
They slowly stained a blue red with iodin. At some places at the 
surface the same glistening colorless substance noted in the potato 
grains was ai)|)arent. 

rBuU.-J02J 




Fig. 4.— Raw starch grains that have l)oon digested 38 hours: o, corn- 
starch showing clefts at what wore tlie edges of the grain; 6, wheat 
starch grain, side view, showing oyslcr-sliell-hke opening and pittings; 
c, arrowroot starcli grain sliowiiig corrosions; d. potato starch grain 
staine<l with iodin, showing colorless reverted amylodextrin and no 
starch at the small end. 



24 

DIGESTION EXPERIMENT NO. 2. 

The conditions were the same as in experiment No. 1, except that 
mah extract was siibstitutetl for saHva and the mixture was dio:ested 
at 55° C instead of at 40° C. The results obtained were practically 
the same as with saliva. 

DIGESTION EXPERIMENT NO. .3. 

In this experiment potato and corn starches were used, each sample 
beinjz; treated with 25 cubic centimeters of malt extract and 50 cubic 
centimeters of distilled water added to 1 o;ram of raw starch. The 
samples were dig^ested at 55° C. for 24 hours and were tested with 
FehHn<;'s solution according; to the method outlined on pa^je 21. 

At the end of 24 hours the tests showed that about 10 i)er cent of 
the potato starch and about 27 j)ercent of the cornstarch were changed 
to maltose. 

From these tests with raw starch it seems fail' to conchuh* that I'aw 
starch diji;(»sts slowly, and that starch grains that are cracked (H(i;est 
more ra])idly than those that are sohd. 

There seems to be evidence furnished by the exjieriments in su|)|)ort 
of the theoi'v that the outer layer of starch i^rains is more (hflicuh of 
(Hircstion than the iimer layers. In the llattened starch <j;rains of corn, 
rice, wheat, and barley the outer hiyer seems to be weakened at the 
edges, so that while the rest of the surface* is bein*; corroded slowly 
more rapid solution takes \)\i\vo at theed,i::es and alon^ the (Tacks on the 
insi(h\ In the rouncU'd, lUKTacked irrains of j^otato and arrowroot 
starches there are no easy |)()ints of attack. The inside is ioo dense 
for- ra|)id euouirh (hlfusion inward of the (hastase and outward of the 
(h^estion prochicts, and the outcn- layer is practi<'ally homogeneous 
exc(>pt that it is thinner at the small end in the potato starch grain 
l)ecause the hilum or point of origin is at this end. TT(Mice it is that in 
this grain the outer layer dissolves first at the snuill end. 

The colorless substance remaining at the end of the digestion 
period may be reverted amylodextrin, since digestion is very slow. 

-ludging by the results of the digestion tests, and from their normal 
condition and appearance without treatment, the six kinds of starch 
studied naturally grou}) themselves into two divisions, namely: 
Wheat, corn, barley, and rice starches and potato and arrowroot 
starches. 

In the case of the wheat, corn, barley, and rice starches the natural 
spherical form has become flattened. The starch grains show internal 
cracks and a red-purple stain with iodm on standing. The digestion 
is apparently more rapid than with the second group. 

In the case of the potato and arrowroot starches a rounded form is 
retained; normally no internal cracks are visible, and a blue-purple 

[Bull. 2021 



25 

stain is obtained with iodin -which does not become red purple on 
standing. The digestion is very slow. 

The starch grains of the first group are produced in the crowded 
cells of seeds and hence their flattened form. In these cells the loss 
of water on maturing is great and hence the cracks. 

The starch grains in the second group are produced in less crowded 
cells, and are not flattened. The drying at maturity is much less and 
so there are fewer cracks. 

EXPERIMENTS WITH STARCH COOKED BELOW THE BOILING 

TEMPERATURE. 

That starch may be cooked it is essential that eYerj starch grain be 
brought into contact with water of at least 60° to 80° C. (140° to 178° 
F.). If it is to be cooked below boiling temperature, this is best 
accomplished for quantitative work by simply adding boiling water 
to a mixture of starch and cold water and slinking. But a paste can 



x~§ 





a, 6 c 

Fig. o. — Potato starch magnified 87 times, showing successive stages in paste formation: Changes that 
come with the absorption of hot water— a, raw starch; x, hilum or point of origin; 6 and c, intermedi- 
ate stages, showing granulation beginning at the point of origin; d. condition in unboiled paste; y. outer 
layer; z, folds in outer layer. 

be made equally weU by the more common method of })ouring the 
mixture of starch and cold water mto boiling water while stirring. 

The same starches were used as in the previous tests. "\Mien the 
pastes were exammed with the naked eye, marked differences were 
noted between potato and arro%\Toot starch on the one hand and corn 
and wheat starch on the other. The potato and arrowroot pastes 
were colorless antl transparent, ^\■hile tlic others were whitish, more 
opaque, and appeared to be less thoroughly cooked. In all cases the 
pastes made without boiling separate on standing a short time, the 
starch sinking to the bottom. 

When examined with tlie microscope it was apparent that the 
starch grains were much swollen but were not broken. The transition 
from the raw to tlie swollen condition can be observed with the 
microscope if the paste is made by using all cold water and gradually 
raising the temperature. The absorption of water by the starch 
grain begins at the point of origin. It shows as a granulation which 
gradually works its way outward (cee fig. 5) until nothing solid but the 

[Bull. 2021 



26 






outer layer is left in the potato anil arrovrroot starch strains. Tn the 
corn and wheat some of the (z;ranular material also remains in solid 
form (see lig. ()). It is probably this which causes the o})acity in 

these pastes. The 
outer layer is 
ureatly stretched 
and, if the paste is 
cooled, collapses. 
Sometimes t hese 
collaj)sed forms arc 
hard to recoij^nize 
as such. Tn the 
wheat this is es- 
pecially true. The 
edo;es of the flat- 
tened form curve 
, ., , , , , over on themselves 

Fifi. 0. — Siaroh prains fromxinhouwl psistos: oand o, wheat staroh — n.col- 
liipscd swoUon };r;uii with cdKt's curved over and -sulid Kf^iiiil;!!' i»;»ti'- SO tliat I lu^ rCSUlt 
rial; h. the same j,'rain flattened out by heating on tlie liol-waler sta^'e; /)ft(»ii looks more 
c. collapseil swollen ^'rain of cornslareh showhiK' solid jiranular material. 

like a star than a 
ball (see i\<^. 0, a), but by usinu; a hot-wat(M' sta»j:e (see p. 10) on the 
micr()scoj)e these forms can be seen to unroll <i;radually (see fi<i;. (>, b) 
ajid to stretch out to their ori<i;iiial swoIUmi foriu. 

Details of tlie artilicial dii^estioii expcriuuMits follow which were 
made with the starch pastes cooked below boilimi' teinj)erat ure. 

DKiKsriON KXn.KI.MKXr NO. 1. 

In this test potato, arrowroot, corn, and wheat starches were tised, 
each sort beinf^ made into a paste by a(hlinu' boiliinj; water to a mix- 
ture of starch and cold watiu' and shaking. rn(b,i:;ested samples and 
samj)les dio'csted for different lengths of time with malt diastase were 
treated with small, mi'dium. and hii;<i;e amounts of io(hn and the color 
HMctions n.oted as a means of jud_ii:in,<2; of dilf(U'ences attributable to the 
several sorts of starch and the len<;th of the diirestion period. The 
potato and arrowroot starches and th(> corn and wheat starches are 
<:;rouped toi^ether. b(>cause in (>acli case these starches ^nnxe the same 
color I'eactions. 

T.\HLt; J. — Diiicstibility (is indicnttd hij indiii hsi of diffcrnit starchca cool'rd below 100° C. 



Titne of digestion of slarch. Anioiint of iodin usp(: 



Color pro<hiced with iodin in — 



Potato and arrowroot 
starch. 



Corn and wheat starch. 



\o difrestion Little I'lire Miie Old rose. 

Do .Mediinii <lo I'tirc hhic. 

Do Kxcess (Jreeii " (inu-n. 

minutes lattle Cure blue Old rose. 

Do Medium do Pure blue. 

Do Much Puri)le Purple. 

" Creen color is due to the blue of the roaciion with tlie yeUow of the iodin. 

tPul!.2021 



27 



Table 1. — Digestibility as indicated by iodin test of different starches cooked below 

100° C. — Continued. 



Color produced with iodin in — 



Time ot digestion of starch. Amount of iodin used, p^^g^to and arrowroot I ^.^^^ ^^ ^^.^^^^ starch. 

starch. 1 



3 minute.'! 

Do... 
Do... 
Do... 

4 minutes 

Do... 
Do... 
Do... 
Do 



Little Pure blue. 

Mediimi Purple — 

More do 

Much , Ked 

Little I Pure blue. 

Medium Purple. . . . 

More I do.... 

Still more Red 

Much ' Brown 



eminutes Little [ Purple. 

Do 1 Medium Red... 

Do More ! do. 



Do 

Do 

8 minutes. . 

Do 

Do 

Do 

10 minutes. 

Do 

Do 

20 minutes. 

Do 

6 days 

"Do 

Do 

7 days 



Still more .....do , 

Much 

Little 

Medium 

More 

Much 

Little 

Medium 

Much 

Little 

Medium 

Very little 

Little 

Medium 

Any amount 



BrowTi 

Red 

Brown 

....do 

....do 

....do 

do 

do 

Yellow (iodin color) . 
....do 



Old rose. 
Pure blue. 
Purt)le. 
Red. 
Old rose. 
Pure blue. 
Purple. 
Red. 
Brown. 
Old rose. 
Pure blue. 
Purple. 
Red. 
Brown. 
Old rose. 
Blue. 
Red. 
Browii. 
Old ro.se. 
Blue. 

Blue brown. 
Old rose. 
Blue. 
Old rose. 
Brown. 

Yellow (of iodin). 
Do. 



TliC periods of tiino in the first coluinii represent the miinber of 
mmutes required to pro<hire the eoior stages indicated in tlie third 
and fourth cohunns. That is, the red color shows at tlie end of 3 
mmutes, the bro\\ii at the end of 4 minutes; the red disappears at the 
end of 10 minutes, and tlie brown at tlie end of 20 minutes. It takes 
6 days for the blue connected with the old rose color to disappear. 

The change of color that comes from the varying anioiiiits of 
iodin used is no doubt due to the presence in the tligestion mixture 
of various compounds which stam diirerently with iodin and which 
have for it various degrees of aflinity. The colors given in column 3 
for digesting potato ami arro^^Toot starches may be explained as 
follows: The blue indicates either starch or Syniewski's amylodextrin, 
and the brown and red are two other d(>xtrins, called li<>re for the 
sake of simplicity according to the color they take with iodin, red 
dextrin and brown dextrin. The starch and amylodextrin have 
the greatest aflinity for the iodin, and only after more than enough 
iodin has been added to stain blue all the starch and amylodextrin 
present can any permanent red or puri)le be obtained. Alter this, 
if there is enough red dextrin present to dominate the blue, we see 
this color; otherwise we have the purple mixture. All the red and 
blue dextrin having united with iodin, the brown dextrin begins to 
show, anil if it is present in large enough (piantities the brown hue 
alone appears. As digestion proceeds, first the blue, then the red, 
and then the brown dextrin diminishes in volume and disai)pears. 
With the corn and wheat stardies there is the same series of color 

[Bull. 2021 



28 

changes, but in acklition tliere is another series which, especially 
with this method of cooking, often conceals the one corresponding 
to that of the potato and arrowToot starches. In order to get the 
gradation of color noted in columns 3 and 4 care must be taken to 
add enough iodin and to do it slowly. The "old rose" differs not 
at all in color from some mixtures of blue and red dextrin with iodin, 
and heretofore has evidently been taken for the same, but as shown 
here the compound stained old rose has a stronger allinity for iodin 
than the blue dextrin, while red dextrin has a less aflinity. The old 
rose always appears before the blue with these starches if iodin is 
added slowly. Moreover, this red is not quickly changed to browm 
by digestion, as is the red dextrin, but continues for days rather than 
for minutes. 

However, it must not l)e concluded, because it takes corn and 
wheat starches over 500 times as long as potato and arrowToot 
starches to reach the point in digestion where they no longer color 
iodm solution, that the relative tiigestibility of tli(» two pairs can be 
expressed by the ratio 1:500; for the part of the paste which is slow 
to digest represents only a small percentage of the whole, as is in- 
dicated by the shght color in the test tubes aft(>r 20 minutes' digestion. 
Moreover, there are probably factors, such as the accumulation of 
by-products, which make the (Hgestive j)ro('ess increasingly diflicidt 
as time progresses. 

When exaniin(Ml wilh the microscope it is seen that the swollen 
starch grains before digestion give to the })aste the blue color with 
iodin, but after a minut(>'s digesti(m this color is seen also in the 
solution outside of the gi'ains without any :i{)parent l)reak in the outer 
layer. ^^Tien most of the bhie substance has diffused out, this outer 
layer is seen to be stained red. It persists until the red dextrin is 
the domhiant compound in solution, and then (luite quickly disap- 
pears. After the solution of the outer layer the digesting i)otato and 
arrowroot mixtures are fr(>e from solid substance. The line granular 
substance of the corn and w heat starches, however, continues undis- 
solved. The particles <ire so tiny that they i)ass through a filter 
})aper. They stain old rose with iodin and undoubtedly explam the 
long continuance of that coloi- in the test-tube examinations. 

DKiKsriOX EXPERI.MEXT X(). 5. 

in this exi)erinient arrowroot, })otato, wheat, and corn starches 
were used, unboiled ])aste being prepared from each as in digestion 
experiment No. 4. When the pastes, after digestion with malt di- 
astase, were tested with Fehling's solution the arrowroot showed 
71 to 73.5 per cent maltose, potato 72 to 74 per cent, wheat 63.8 to 
66 per cent, and corn 58 to 60 per cent. 

A comparison of these figures with those given as the result of 
digestion experiment No. 2 shows that imder the same digestive 

(■Bull. 202] 



29 

conditions, but in one forty-eighth of the time, unboiled pastes of 
potato starch yield about seven times as much maltose as does the 
raw potato starch, wliile cornstarch paste yields only about twice 
as much as the raw cornstarch. However, it does not necessarily 
follow that potato-starch j)aste is 336 times as digestible a;j raw 
potato starch, or that cornstarch paste is 96 times easier of digestion 
than raw cornstarch. 

From the work on starch cooked below })()iling temperature it 
seems fair to conclude that starch pastes need not ])e boiled to make 
them much easier of digestion than raw starch. These experiments 
show yet another difference between potato and arrowroot starches on 
the one hand and corn and wheat starches on the other (see pp. 24, 25). 
There is present in corn and wheat starches a substance not found 
under these conditions in potato and arrowroot starches. It has 
the following properties: It stains old rose color with iodin, it has 
more aflinity for iodin than starcii or blue dextrin, it is soluble in 
warm water, and it (Ug(»sts very slowly. 

It would seem, tli(>refore, that it cau not l)e the amyhxiextrin, 
which Meyer thinks is present in those grains that stain red with 
iodin, for that is soluble in warm water and easily digestible. Nor 
can it be Syniewski's maltodextrin I (the erythrodextrin of others) 
which stains red with iodin, but is very easily soluble in warm water 
and very easily digestible, and which has less affinity for iodin than 
blue dextrin. It is evidently not a reversion product, for its color 
shows in the raw starch grains. It has some of the ])roj)erties of 
Meyer's rt'-amylose, in that it stains red with iodin, and is difficult 
of solution and digestion. However, this substance is not distributed 
in the starch grains as he believed <^r-amylose to be distributed. 
For the sake of simplicity this starch constituent may be called 
rose amy lose. 

Th(^re is j)resent in all the swollen starch grains tested a substance 
with the following properties: It stains blue with iodin, it digests 
rapidly even if the outer layer has not been broken, and it apparently 
constitutes the whole of the potato and arrowToot starcli grains 
except the outer layer and all of the inside of the corn and wheat 
starch grains except about 10 pcv cent which is rose amylose. It 
is evidently the /^-amylose of Meyer, the granulose of others. For 
the sake of uniformity this portion or constituent of the starch grain 
may be called blue a]iiylos(\ 

It was further (>videnl from the digestion experiments reported 
that the outer layer of the starch grain is not broken or dissolved 
in paste formation below lOO"" C. It stains red with iodin and is 
perhaps the cause of the slight i)urple hue seen in raw potato and 
arrowroot starches so stained: it is harder of digestion than blue 
amylose; it is easier of digestion than rose amylose, and it seems to 

[Biill.2li2] 



30 

differ from the other hiyers in more resj)ects than in density, since 
even in its stretched condition it stains red instead of bhie or rose 
with io(Un. Perhaps it is formed as a residt of changed contUtions 
in the cell sap that come after the end of the growth period in the 
plant, and wliich might affect the outer layer most because of its 
position even tliough the whole grain be porous. The substance of 
this layer may be called red amvlose. 

EXPERIMENTS WITH STARCH BOILED 5 MINUTES. 

The ])astes which are made by boiling the starcii for 5 minutes 
are more homogeneous and do not show the tendency to separate 
wliich is noted with uid>oiled pastes. The cereal starch pastes are 
still not clear wlien thus cooked though the others are. 

The microscope shows that large munbers of the starch grains 
have broken, corn and wheat starches showing a nudtitude of 
fragments of broken outer layers and more or less granulated inner 
])arts. With tlie arrowroot and |)otato starclies the outer layers 
are more [)ersistent and, collapsed as they are, it is often hard to 
see tlie breaks, but with iodin it is easy to sec by their greater trans- 
])arencv that they have lost their inner parts, wlii(di are more uni- 
forndy solid)l(' than in the other starches. 

DKiESTlOX KXlM'.lv'IMKXTS XOS. () .VXD 7. 

In thes(> experiments both methods and conditions were the same 
as in (Hgestion exj)eriments Nos. 4 and o e.xcept that the pastes 
were boiled o minutes, and the results obtained were on an average 
tile same as those given on j)ages 2()-o0 for tlie experiments with the 
unboiled })astes, all of ^\hich are included in Tables 2, 3, and 4 for 
purposes of comj)arison. 

Tlu> conclusion may therefore l)e drawn that brief boiling, while 
it breaks many starch grains and makes a better paste, does not 
ap|)recial)ly improve the digestil)ility of the starch. 

EXPERIMENTS WITH STARCH BOILED MORE THAN 5 MINUTES. 

To pre\('nt the formation of skin and the loss of water all the 
sani])les w(>re ])oiled in a iiask with a return condenser. 

With continued boiling the ))astes gradually change in ai)pearance 
and character. At the end of .'5 hours they are distinctly more 
homogeneous, less viscous, and more transparent, and they also 
acquire^ a slight yellowish tinge. The taste is changed from one 
more or less suggestive of the odor of boiling laundry starch to 
one very slightly suggestive of caramel. This slight change of 
color and taste is not due to any ordinary scorching, because there 
is n a]>i>arent sticking of the ])aste to tlie bottom or sides of the 
ilask. The microscope shows a gradual diminution in the solid 

nuiterial, i. e., the outer layers and granular substance ])resent. 

rniiii. I'd')! 



31 



DIGESTION EXPKKI.MEXT XO. 8. 

Potato and arrowroot starches wore used for this experiment, and 
the pastes were boiled for 3 liours and were treated with malt diastase 
in the usual way. The results obtained are summarized in the fol- 
lowing table, which shows the time required for the brown dextrin 
reaction to disappear from the digestion mixture. For purposes of 
comparison the results of similar tests with unboiled starch and with 
starch pastes boiled for o minutes are also included. 

Table 2. — Changes brought ahout in potato and (irrotrroot stnrchrs by long continutd 

cooking. 



Time required for disap- 
pearance of brown dex- 
trin reaction. 



Time required for disap- 
pearance of l)rown dex- 
trin reaction. 



Kind of starch. 



With '^^''^^ 



With 
pastes 
boiled 3 
hours. 



Kind of starch. 



With 
unboiled 
pastes. 



With With 

pastes pastes 

boiled .") boiled 3 

minutes, hours. 



^^nuteli. Minutes. Minutes. 

I 10 -201 185-20.', 

\m-2ll 10i-205 j,^j^,,^_ 



Average. 



20 



20 



Minute.t. Minutes. Minutes. 

10-21 20-2.') 18-20 

17-18 1 10-23 10-2U 

20-2.') ' 10-24 

23-25 20-25 



Average. 



21 



211 



20i 



DIGESTION EXPERIMEXT XO. 9. 

In this experiment arrowroot starch was used. The paste was 
boiled for 3 hours, when, after digestion in the usual way and 
tested with Fehling's solution, it was found that 71 to 73.5 per 
cent of starch had been converted into maltose — the same quantity 
which was notccl in the case of unboiled arrowroot jniste (see p. 28). 

It would therefore ap])ear from the results ol)tained in the diges- 
tion experiments Xos. 8 ami that ])otato and arrowroot starches 
are not mad(> more digestible by boiling. 

DIGESTIOX EXl'KKI.MEXT XO. 10. 

The cornslarch used was a commercial ])reparation designated '"A." 
The unboiled pastes and samples, boiled from ."> minutes to 3 hours, 
were prepared, digested in th(> usual way. and tested with iodin. 
The results which were obtained are as follows: 

Table 3. — Changci brought about in cornstarch by long continued cooking. 





Time required for disappearance of red 
dextrin color reaction with paste. 


Time required for disappearance of brown 
dextrin color reaction with paste. 


No. of test. 


Un- 
boiled. 


Boiled Boiled 
5 min- 30 min- 
utes, utes. 


P,^j'!;f Boiled 
^^;;i^"- 3hours. 


,.„ 1 Boiled 
toiled. '^^-jy- 


Boiled 
30 min- 
utes. 

Hrs. 
2. 3-2. 5 
2. 5-3. 3 


Boiled 
45 min- 
utes. 

Ilrs. 
'i."2-i.'7' 


Boiled 
3 hours. 


1 


Min. 
7-16 
5-14 


Min. 
0-12 

8-11 


Min. 
7 -10 
7. 5-12 


Min. Min. 
7- 9 8. 5-0. 5 
6-13 ; 8 -12 


IJrs. 
23-24 
24-25 


Hrs. 

28-29 

26-27 


Min. 
51-50 


■{ 


9-12 
10-18 




21-22 
21-22 






4 






























.\verage . . . 


N 11 10 


9.25 


11 0.5 


23 1 27 


2J 


'■1 ■'' 



[Bull. 202] 



32 

Comparison with Tiil)l(' 1 makes the results of this experiment 
more easily understood. The first part of Table 3 corresponds 
with the results given in Table 1 with samples cooked 10 minutes 
and shows the time which it takes for the disappearance of the red 
dextrin of the easily digested portion. As explained in connection 
with Table 1, it is impossible to see the transition at the end of 20 
minutes with corn and wheat starches. 

The relatively rapid digestion of the rose amylose (23 hours instead 
of over 7 days, as noted in the experiment reported in Table 1) is 
doubtless due to some peculiarit}^ of the preparation of cornstarch 
used (see pp. IS, 27). 

From the experiuiental data it would appear that the l)hie amylose 
of corn, as of potato and arrowroot starches, is not made more easily 
(Hgestihle by long boiling. The rose aniylos(> of cornstarch is appar- 
ently nui(I(^ more quickly (Hgestihle by long ])oiling because some of 
it is changed to blue auiylos(>. 

DKJKSTIO.V EXPKin.MF.Xr X<). 11. 

In this, as in the preceding test, conuuercial cornstarch d(>signate(l 
''A" was used. The pastes were boiled for o minutes, 30 mimites, 1 
hour, aud .') hours, respect i\'ely, and dig(>sted for 30 mimites. When 
t(\sted with F(>hling's solution it was found that a sample l)oiled 5 
mimites gave 5!) to 61 .2 })i'r cent maltose: the ])aste boiled ."!() mimites 
00 to 02.1 per cent; the })aste boiled 1 hour OiJ.o to Oo.l ])er cent, and 
tlie ])aste boiled 3 hours 02.1 to ().'). 7 per cent, as compared with 5<S to 
00 per cent maltose in the case of unboiled pastes. Judged by the 
data reported there is a (hstinct increase in the percentage of maltose 
formed in 30 minutes' (ligestion of cornstarch pastes, boiled 1 and 
3 hours, over those cooked short(>r lengths of tini(>. 

DICESTION' KXPHHIMFA'T XO. 12. 

Samples of pastes made from wlieat starcli wdo boilml for 1 and 3 
hours, respectively, and digested with malt <>\tract in tlie j)roportion 
commonly used in the experiments, namely, 2.1 cubic centimeters of 
malt extract and 50 ciil)ic centimeters of water to 1 gram of starch. 
^Mien tested with iodin all the samples re(|uired over 7 days for the 
(hsa])pearance of the characteristic d(>xtrin color, the only noticeable 
difference being that with the pastes boiled for the longer period there 
was less substance left undigested at the end of 20 minutes than with 
th(> other pastes. 

DKJESTIOX EXPERI.MEXT NO. 13. 

In this experiment wheat starch was used and the paste was boiled 
for (hfl"erent periods of time. The proportion of malt extract to starch 
used for digesting the paste was ten times greater than in the majority 

[Bull. 202] 



33 

of the tests, 50 cubic centimeters of malt extract and 20 cubic centi- 
meters of water beinij used to 0.2 (rram of starch. The resuhs obtained 
upon testin^:; with iodin are shown in the followino; table : 

Table 4.— Changes brought about in wheat starch by cooking for dljfc rent lengths of time. 



Kiiui of starch. 


Time required for disappearance of red 
dextrin color reaction with paste. 


Time required for disappearance of 
l)r(nvn dextrin color reaction with 
paste. 




Un- 
boiled. 


Boiled .5 
minutes. 


Boiled 1 
hour. 


Boiled .3 
hours. 


Un- 
boiled. 


Boiled ,5 
minutes. 


Boiled 1 ' Boiled 3 
hour. hours. 


W.'ieat starch: 

First test 

Second test 

Average 


Mmules. Minutes. 
3-.') 5-7 


\nnutes. 

.5-6 

.5-6 


Minutes. 

:v>-.5 

4-."> 


Hours. 
4-18 
4-18 


Hours. Hours. 

4-18 4-18 
4-18 1 4-lS 


Hours. 
1-li 


A\\ 5 


oil 4a! ' 



Xo diflVrence is a])])arent from the data included for the dis- 
apj)earance of the brown dextrin color reaction, owini!; to the fact 
that the end points of the reaction occurred some time durintj the 
ni<;ht.\\hen it was not possible to make the necessary observations. 
There was, however, as in experiments Nos. 6 and 8, a noticeal)le 
difference between the })astes boiled not more than o minutes and 
those boiled 1 and W hours, as shown by the (piantity of im(liij;ested 
material left at the end of 20 minutes. 

It secMiis fair to con(dude, tluM'efore, that increasin*]; the [)r()portion 
of diastase to starch shortens the time re(piir(>d for both stat!;es of 
digestion. Lon<2; boilin*: makes the rose amylose of wiieat starcli 
more easily di<;estil)le, as is the case with cornstarch, and as previ- 
ouslv found has no etfect on the blue amvlose. 



DKJESTION' KXPEKl.MEXT XO. 14, 

In this test wheat starch ])astes boiled for dilferent periods of time 
were used. The pastes were digested with the ordinary proj)ortions of 
malt extract, namely, 25 cubic centimeters malt extract and 50 cubic 
centimeters water for 1 gram of starch. On testing with Fehling's 
solution it was found that the uuhoih'd pastes showed (54.8 to 68.3 
per cent maltose in the first test antl ().3.8 to 66 per cent in the second 
test. In the case of the samples boiled 3 hours 64.8 to 67.3 per cent 
maltose was noted in the first test, and 67.5 to 70.5 per cent in the 
second test. 

The increase in (Ugestihility due to long boiling, as indicated by 
the production of maltose, is small, being even less than was noted 
with cornstarch. There seems to be a discrepancy between the results 
of the iodin test (ex))eriment No. 13) and the test with Fehling's solu- 
tion. The iodin test would indicate that wheat starch is more diffi- 
cult to digest than cornstarch, while the test with Fehling's solution 

[Bull. 202] 



34 

would iiulicalc the reverse. Further iiivestiijjatioii is recjuired, there- 
f()r(>, l)el'ore general couchisioiis eaii be drawn regarding the relative 
digestibility of wheat and eorn starches. 

In general, the conclusion seems warranted, from experiments Nos. 
8 to 13, that pastes made from potato and arrowroot starches, 
which have been boiled even for 3 hours, do not digest more quickly 
than similar pastes made with hot water but not heated to the 
boiling point. Corn and wheat starches are rendered somewhat 
more digestible by long boiling owing to the gradual change of the 
rose aniylose which they contain to blue amylose. A long cooking 
j)(>riod is, however, necessary, as the change to l)lue amylose is not 
coni])lete after 3 hours' boiling. 

EXPERIMENTS WITH STARCH COOKED IN A WATER BATH OR 

DOUBLE BOILER. 

AVheii starch paste is cooki^l in a water bath or double boiler, in 
which the water is kej)t at boiling temperature, the starch grains do 
not break but settle to the bottom of the vessel in which the paste is 
cooked. 

DKilOSTION EXPERIMENT XO. 1"). 

Cornstarch was used in this experiment and was cooked in a boiling 
water bath for 1 hour, using a flask with a return condenser. The 
temperature of the starch mixture itself was found to be 95° C. 
When tlu> mixture, after digestion in the usual way, was tested with 
iodin it was found that the brown dextrin disappeared in 5 to 18 
hoiu's as compared with 23 hours in the case of unboiled paste and 
'2\ hours in the case of starch boiled 30 minutes, the values which 
were obtained in previous tests. 

T\\v conclusion is drawn, therefore, that cooking cornstarch below 
100° C., as in a. water bath, increases its digestil)ility, l)ut not so ra])idly 
as cooking at 100° C. 

EXPERIMENTS ON THE DIGESTIBILITY OF STARCH BY DIFFER- 
ENT DIASTASES. 

It- was felt that it was desirable to compare the digestibility of 
starch by malt extract with that by other diastases, and so experi- 
ments were undertakt^n in which ptyalin and taka diastase were used. 

DIGESTION EXPERIMENT NO. 1(). 

Wheat, corn, and ])otato starch pastes were prepared in the usual 
way and boiled for 1 minute. One sample of wheat starch was 
also boiled for 3 hours. The samples were digested with saliva 
(see p. 10) and then tested with iodin. In the case of potato starch 

[lUill.202] 



35 

boiled for 1 minute the brown dextrin reaction disappeared in 8 to 
12 muiutes, and with corn and with wheat starch, cooked the same 
length of time, in 24 hours in each case, while with wheat starch 
cooked 3 hours 3 to 18 hours were required. The corn and wheat 
starches showed the slow disappearance of rose color, but the potato 
starch did not. 

The ptyalm of saliva, like malt extract, according to the data 
reported, digests potato starch quickly and corn and wheat starch 
slowly. When wheat starch was boiled for 3 hours it was digested 
more quickly than when boiled for 1 minute. 

DIGESTION' EXPERIMENT XO. 17. 

Wheat, corn, and potato starch ])astes were prei)ar<Ml, using 1 per 
cent of starch, and were cooked on a water bath for one-half hour. 
The cUgestibility was tested with taka diastase, using equal quanti- 
ties of the diastase solution (sec p. 20) and starcli. The digt'stion 
was carried on at a temperature of 65° C. On testing with iodin it 
was found that the brown dextrin reaction disappeared in 12 minutes 
with potato starch and with both the wheat and corn starch pastes 
in 20 hours. The ros(» color appeared with the corn and wheat 
starches, but not with the i)otato starch. 

From the tests reported, therefore, it wouhl a])pear thai (Hastase 
of animal origin, ptyalin, and taka diastase, a diastase derived from 
a fungus {Eurotium oryzse), show the same reaction on blue and rose 
amylose as does the malt diastase obtained from barley. 

It was thought possible tliat the results obtained in the foregoing 
experiments might be due in part to the i)articular sann)le of starch 
used, so efforts were made to obtain other samples and to use them 
in check cxpiMiments as follows: 

EXPERIMENTS ON THE DIGESTIBILITY OF DIFFERENT PREPA- 
RATIONS OF THE SAME KIND OF STARCH. 

DIGESTION EXPERIMENT XO. IS. 

Starch ])repared from new ]K)tatoes, from old potatoes, and from 
much-sprouted potatoes, and a commercial potato starch ])repara- 
tion were used in this experiment, none of the pastes being boiled. 
The color clianges found on testing with iodin were the same as those 
reported for i)otato starch in Table 1 (p. 26), and there was no 
indication of rose amylose in any of tlie juTparations. The time 
required to reach the achromatic jioint was ap[)arently the same with 
all the samples. Tlie brown dextrin reaction disappeared with tlie 
starch made from new i)otatoes in 18 to 19 minutes in th(^ (irst test 
and in 22 to 23 minutes in the second test; with starch made from 

[Bull. 202] 



36 

old |)otatoes in 20 to 25 iniimtes in the first test and in 20 to 21 min- 
utes in the second; witli starch made from much-sprouted potatoes 
in 16 to 18 minutes hi the first test and in 15 to 17 minutes in the 
second, and with paste made from conunercial })otato starch in 17 to 
IS minutes in the first test, 20 to 25 minutes in the second test, and 
in 24 to 25 minutes in a third test. 

On the basis of the reported data it would seem fair to conclude 
that rose amvlose is not present in potato starch in such quantities as 
to a|)preciably aiTect its diii-estibihty. 

DHIKSTIOX EXPERIMENT NO. 19. 

Unboiled pastes were made from cornstarch prepared in the laJ)ora- 
tory from corn meal, and from three commercial cornstarches desig- 
nated "A," "B," and ''C," res])ectively. The results, which were 
obtained on testing- with iodin, after digestion in the usual way with 
malt diastase, are shown in the table which follows: 

Tablk •"). — (_'<)iii parisou of (/!<iisl ihllitii <if iinlxilUd jxislis iiiddr jnim CDnisfdrcliis of 

diljiiciit origin. 



Kind of starch. 


Tinu- re- 
(luired for 
disaiijiear- 
aiico of red 

dextrin 
color reac- 
tion. 


Time re- 
(inired for 
disappear- 
ance of 
Ijrowii dex- 
trin color 
reaction. 


Kind of st 

Comniercial c 

First test. 

Second test 

Connnercial c 

First test . 
Second test 


ari-li. 
irnstarch 


Time re- 
(piired for 
iiisai)])ear- 
ance of red 

dextrin 
color reac- 
tion. 


Time re- 
quired for 
disapjH'ar- 

ance of 
brown dex- 
trin color 

reaction. 


•Starch made from corn 
meal: 


Mimit/'s. 
(1-14 
5-13 

7-1(1 
5-14 
0-12 
10-18 


S-23 
J<-23 

23 24 
24-25 
21-22 
21-22 


3f/nutes. 
7'i-9 
"i-'J 

(i-15 
(1-9 


Hours. 






irnstarch 




Commercial cornstarch 
"A:" 
First test 


KIS 


Third test 

Fourth test 







As will be seen from the table, a great diirerence was notetl with the 
four samples of starch in the time riHjuired for the last stage of diges- 
tion, i. e., the (lisaj)])earance of the In'own dextrin. No explanation 
of this diflerence has been found, and the ])oint is an important one 
which requires further attention. 

From the recorded data it seems fair to conclude that ])()th hUie 
amvlose and rose anulose are normal const it iients of cornstarch. 



DKiF.STloN KXPERIMEX'P XO. 20. 

Unboiled pastes were ]>repare(l from a commercial wheat starch, 
from a sample of wheat starch jH-epared in the laboratory, and from 
wheat starch in the form of wheat fiour, and were tested with iodin 
after digestion in th(> usual way. In all cases digestion occurred in 
two stages, the old-rose color appearing. 

[Bull. 202] 



37 

The conclusion was reached, therefore, that both bhie and rose 
amylose are normal constituents of wheat starch. 

In general, it seems fah to conclude that the results obtained in 
experiments Nos. 4 to 18 on the composition and digestibility of 
wheat, corn, and potato starches are normal and are not ascribable 
to the different methods used in isolating the starches. 

ADDITIONAL ATTEMPTS TO IDENTIFY ROSE AMYLOSE IN PO- 
TATO STARCH. 

Since recognizable quantities of rose amylose in wheat and corn 
starch are found to vary with experimental conditions, it was thought 
that it might possibly be detected in potato starch under conditions 
unfavorable for digestion and three experiments were made to study 
this point. 

DIGESTION EXPERIMENT NO. 21. 

A sample of unboileil paste was ])repared and digested at 61° C. 
instead of 55° C. On testing with iodin it was found that the time of 
digestion was slightly increased, but there was no indication of rose 
amylose. 

DIGESTION EXPERIMENT NO. 2-. 

Unboiled potato starch paste was treated with a fifth as much dias- 
tase as w^as used in the majority of the experiments, the quantities 
being 5 cubic centimeters of malt extract and 50 cubic centimeters 
of water to 1 gram of starch. The time recpiired to reach the achro- 
matic point was increased from 20 to SO minutes, but there was no 
indication of rose amylose. 

DIGESTION EXPEKIMENT NO. 2.S. 

Raw potato starch ground w iih glass fragments to break the outer 
layers of the starch grain was tested with iodin and it was apparent 
that digestion occurred much more ra])idly than with unbroken raw 
starch, but much more slowly than with starch grains which had been 
swollen with hot water. \Vhen examined with the microscope they 
showed that the outer layer was red with iodin whenever it was sepa- 
rated from the inner parts, but the digestion mixture tested with iodin 
in test tubes showed no old-rose color before the blue and no tendency 
of the blue to revert to the okl-rose color on standing. 

The general conclusion seems warranted, therefore, that rose amy- 
lose is not present in potato starch. The red amylose of the outer 
layer of the potato starch grain does not give the same reaction. 

[Bull. 202] 



THE POSSIBILITY OF REVERSION IN WHEAT AND CORN STARCHES. 

AccDidiiig to Maqiienne and Jloiix, reversion does not occur above 
60° C. The following experiment was made to determine whether 
rose amylose is a reversion product: 

DIGESTION' HXPEKIMENT XO. 24. 

Unboiled pastes were made from wheat and corn starches and care 
was taken not to cool the pastes below 61° C, and digestion was con- 
tinued at this temperature. Treatment with iodin showed that di- 
gestion was retarded in both cases, but the results were otherwise 
the sauie as in previous experiments. 

Froui th(> results of this ex])eriuient and the fact that the rose color 
is also api)anMit in the raw graiu on treatuient with iodin, the deduc- 
tion would seem warranted that rose amylose is not a reversion 
])r()(luct. 

Little work was done on the subject of the formation and proper- 
ties of the possible reversion product in connection with the investi- 
ii;ations reported in this bulletin, but enough was accom])lished to 
suiigest that such a reversion product i> formed when raw starch di- 
gests slowly, when starch i)astc> are fro/en, when a >kiin forms on 
stai'ch paste, and when a partially digested starch i)ast(> stands at 
ordinary laboratory temperature. It does not seem to be formed 
when dilute (2 i)er cent ) starch pastes are kept st(»])pere(l for 24 hours 
at ordinary laboratory temperature. 

As regards the properties of this reversion |)roduct, it does not stahi 
with iodin; it is more dillicult of solution and digestion than rose 
amylose, and it dissolves in ])ota:rsium hydroxid, forming a solution 
which after neutralization gives the characteristic blue color with 
iodin. 

This reversion product, which is probably the same as Syniewski's 
reverted amylodextrin, may be regarded as distinct from the red 
amylose and rose-amylose Ixxlics, which are apparently now for the 
first time definitely identified, but it has undoubtedly been ct)id"used 
with them l)y earlier investigators who have variously called it starch 
cellulose, amylocellulose, farinose, and rt-aniylose. Further investiga- 
tions are necessary before definite conclusions can be drawn ;is to the 
(liet(4ic significance of the reversion of starch. 

EXPERIMENTS ON THE DIGESTIBILITY OF TAPIOCA, SAGO, 
PEARLED BARLEY, AND RICE STARCH. 

In coimection with the investigations which have been reported it 
seemed desiral)le to test the digestibility of some other common sorts 
of food starches and accordingly tapioca, sago, pearled barley, and 
ric(> w(>re selected for study. 

[Bull. L'U'.'l 



39 

DIGESTION EXPERIMENT NO. 25. 

Three commercial preparations of tapioca, two of sago, one sample 
of pearled barley, and one of rice were selected for this experiment. 
No attempt was made to free the starch from cellulose or other sub- 
stance, the materials as purchased bein^; simply cooked on a water 
bath until soft. When the starch was dio;ested tlu> proportion of 
malt extract to starch was not definitely fixed, since no attempt was 
made to secure other than qualitative residts. When tested with 
iodin, it was found that the tapioca and sago digested in one stage as 
did potato and arrowToot starch (see Table 1). Rice and barley 
showed distmctly the two stages found in the digestion of corn and 
wheat starches (see Table 1). 

These results, therefore, confirm the conclusions from earlier experi- 
ments that the starches produced in the cereal grains difTer from those 
in roots and stems in that they contain rose aniylosc, and hence are 
more diliicult of complete digestion unlc^ss cooked for a long time. 

CORNSTARCH v. WHEAT STARCH. 

The experiments here reported luive shown that, in so far as there is 
a difrerence, cornstarch digests more rapidly than wheat starch and 
the digestibility of both is ahke increased by long-continued cooking. 
The housekeeper's rule that cornstarch should be cooked longer than 
wheat starch seems to be founded on th(> fact that cornstarch })astes, 
unless cooked .'50 to 40 minutes, have a |)ecnhar Ihivor, disagreeable to 
most persons, which is not noticed with wheat and other starches. 
Efforts were ukuU' to learn the cause of this flavor, but without suc- 
cess, and more work is (Uvsirabk^ on this point. It seems probable that 
the flavor is due to an impurity in the commercial starch which either 
evaporates or is deconipos(Ml on long-continued cooking. 

From the standpoint of Uavor. therefore, cornstarch shouhl be 
cooked 30 to 40 minutes. Moreover, if Pawlow" is right in concluding 
that foods which })lease the palate are more easily digested than those 
which do not, it may be true that th(> long-continued cooking increases 
the digestibility of the cornstarch under natural conditions more than 
would be hidicated by t heart ilicial digestion experiments Iumc reported. 

DEDUCTIONS AND CONCLUSIONS. 

In applying the results of these experiments it must be reinendx'red 
that in general the starches used in the experiments had been s(>parated 
from the other sid^stances with which they are ordinarily combined in 
foods. Hence, hi cooking starchy foods, such as potatoes, it is neces- 
sar}' to consider the time required for the heat to j)enetrate to the 
center of the mass, and to reineml)er that the effect of cooking })otatoes, 
for instance, includes changes in the cellulose ])resent as well as in the 

« The Work of the Digestive Glands. LoikUhi, 1902. 

[Bull. 202] 



40 

starch. Furthermore, the relative digestibihty of two starchy foods 
may (k^pend on other factors than the starch content. For instance, 
the fact that rice is considered easier of di.ij:estion or better tolerated 
than potatoes, an opinion which seems to be based veiy largely on 
experience gained in invalid dietetics, is, if true, probably due to the 
relative digestibility of other constituents than the starch in the foods, 
since rice starch, as shown by the experiments here reported, contains 
the slowly digesting rose amvlose not found in potato starch. It 
nnist l)e remembered, further, that the digestion experiments reported 
were made by artificial rather than natural methods, and that it is 
hardly possible in lal)oratory experiments to ikiplicate all the condi- 
tions i)resent in natural (Hgestion. 

Lee," among others, has given reasons against drawing conclusions 
from test-tube experiments with regard to the limits of digestion. He 
found that when a dialy/.er was used to nuuove the products of dia- 
stase action as th(\v wcvc formed, digestion was carried much further 
than un<h>r usual test-tube conditions. However, such objections are 
not so applicable whei'e the results sought are oidy comparative, as 
was the case in the present work. As suggested earlier (see p. 2S), it is 
probal)l(>that ros(> amy lose would have been found to digest more rapidly 
had a dialyzer Ixhmi emj)loye(l, but such a modification would also aid 
the digestion of hlue amylose, and it is probable that the ratio between 
the rates of digestion of the two woidd not have been much alteretl. 
However, it is reasonable to suppose that there may be physiological 
factors in natural digestion which were not pivsent in these experi- 
ments, which would alter the relative r(>sults obtained from laboratory 
tests.'' Bearing in mind these limitations, it se(>msfair to draw the fol- 
lowing conclusions from the experimental evidence reported witk 
|)otato, arrowroot, taj)ioca, sago, wheat, corn, rice, and barley starches. 

Three substances arc found in raw starch grains, which are designated 
in this bulletin according to the color they give with iodin — blue 
amylose, red amylose, and rose amylose. 

\M\w amylose constitutes tke entire inside of the starch grains of 
potato, arrowroot, ta|)ioca, and sago and !>() per cent or more of the 
inside of the cereal starches studied, luimely, wheat, corn, rice, and 
barley, and is identical with the substance called granulose or //-amy- 
lose by other investigators. It gives a blue color with iodin and in 
th(> solid form found in raw starch it is digested slowly because of its 
dejisity. It takes up water at 60° to S()° C. and forms the sticky 
colloidal substance known as starch ])aste, in which form it is very easily 
digested, l^ong l)oiling, at least to th(> ext(Mit of 3 hours, does not 
make it more quickly digestible. 

".Tour. Physiol., 11 (1890). p. 226. 

&Sce the rcsiill of Ilarcouri's work, ]). 17. 
[Bull. 202] 



41 

Red amylose constitutes the outer layer of the starch grains. 
It ffives a red color with iodin and is more difhcult of dio-estion or 
change in water than blue amylose. The ])resence and the density 
of this layer hinders, though it does not prevent, the digestion of 
raw starch grains. \Mien starch ])aste is made without l)oiling the 
red amylose layer stretches, though it does not break, and in this 
condition is easily permeable and does not interfere^ with the more 
rapid digestion of the inner portions of the grains. When starch 
paste is boiled this layer breaks and a more homogeneous, though 
not more digestible, paste results. 

Rose amylose forms about 10 per cent of the inside of cereal 
starches and is not found in i)otato, arnnvroot, tapioca, and sago 
starches. It gives an old-rose color with iodin and shows a greater 
affinity for the iodin than the blue amylose, takhig the color sooner 
and retaining it longer. It digests more slowly than (Mtherbhie or 
red amylose, and hence it is fair to conclude that the cereal starchcf^ 
are not as readily digested as the other starclu^s studied. Uose aitiy- 
lose is slowly changed by cooking to the blue amylos(» form, hcmce 
cereal-starch pastes are made somewhat more (>asily digestible by cook- 
ing them for several hours. 

There is a fourth substance not found in luitural starch grains 
which has been fre([uently confused with red and with rose amylose. 
It is formed as a reversion ])roduct from solutions of starch and 
has been designated reverted amylodextrin by Syniewski. It is 
found in th(> skin that forms on a starch paste exposed to dry air. It 
does not stain with iodin and is more diflicult of solution and of 
digestion than any of the amylose forms of stai'ch. 

Raw starch, as shown by the ex|)erinunits reported, digests very 
much more slowly than the starch in the forms of a ])aste. 

Starch i)aste made below th(> boiling t<Mni)eraturc of water is as 
easy to digest as that which has Ix'cn boiled a few niiiuites, though 
it is not as homogiMieous. 

Potato, arrowroot, and i)robably tapioca and sago starch ])astes 
are not made more easily digestibl<> by long-continued cooking. 
On tile other hand, the cereal starches are made more easily digest- 
ible by long cooking, though the change occurs very slowly and 
perhaps the increased digestibility is not sullicieutly great to justify 
the trouble, under ordinary circumstances at least, for separated 
starch such as is used in cookery. However, in the case of starch 
still inclosed in cellulose cells, as in many starchy foods, the long- 
continued cooking may be necessary. The commercial j)re])arations 
of cornstarch recpiire 30 to 40 mimites' cooking because of the 
improvcMuent in flavor which results. 

[Bull. 2021 



42 

Skin formation as well as lumps should be avoided in cooking; 
starch — the latt<>r contain raw starch, the former reverted amylodex- 
trin, and both are very slow of digestion. 

The selection of potato starch instead of corn or wheat starch for 
thickening sauces, in accordance with the custom of French cooks, 
is rational, since it contains no rose amvlose and so forms a clearer 
and more digestible sauce, and since it does not require 40 minutes' 
boiling for improvenuMit in flavor, as is the case with cornstarch. 

Increasing th(> |)i'()poft ion of diastase to starch in artiiicial digestion 
experiments markedly hastens the (hgcvstion of both blue and red 
amylose. It would appear, therefore, that eating starchy foods 
slowly, which would of coui'se increase the jjroportion of saliva and 
ptvalin to a given (puuitity of starch, would he of more value from 
the standpoint of digc^stion than would any cooking of starch beyond 
the stage of ])ast(> formation. 

[Bull. 2<I2J 

o 



LIBRARY OF CONGRESS 



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